(l). Primary Productivity of Plants

Introduction

The bodies of living organisms within a
unit area make up a standing crop of biomass. More
specifically, biomass can be defined as the mass of organisms
per unit area and is usually expressed in units of energy
(e.g., joules m-2)
or dry organic matter (e.g., tons ha -1 or
grams m -2).
Most of the biomass in a community is
composed of plants, which are the primary
producers of biomass because of their ability
to fix carbon
through photosynthesis.
This chemical reaction can be described by the following
simple formula:

6CO2 + 6H2O + light
energy >>> C6H12O6 + 6O2

The product of photosynthesis is a carbohydrate,
such as the sugar glucose,
and oxygen which is released into the atmosphere (Figure
9l-1). All of the sugar produced in the photosynthetic
cells of plants and other organisms is derived from the
initial chemical combining of carbon dioxide and water
with sunlight (Figure 9l-1). This chemical reaction
is catalyzed by chlorophyll acting
together with other pigment, lipid, sugar, protein,
and nucleic
acid molecules. Sugars created in photosynthesis
can be later converted by the plant to starch for storage,
or it can be combined with other sugar molecules to form
specialized carbohydrates, such as cellulose.
Sugars can also be combined with other nutrients such
as nitrogen, phosphorus, and sulfur, to build complex
molecules such as proteins and nucleic
acids.

Figure 9l-1: Inputs
and outputs of the photosynthetic process.

The primary productivity of
a community is the amount of
biomass produced through photosynthesis per unit area
and time by plants, the primary producers. Primary productivity
is usually expressed in units of energy (e.g., joules
m -2 day -1)
or in units of dry organic matter (e.g., kg m -2 year -1).
Globally, primary production amounts to 243 billion metric
tons of dry plant biomass per year. The total energy fixed by
plants in a community through photosynthesis is referred
to as gross primary productivity (GPP).
Because all the energy fixed by the plant is converted
into sugar, it is theoretically possible to determine
a plant's energy uptake by measuring the amount of sugar
produced. A proportion of the energy of gross primary
productivity is used by plants in a process called respiration.
Respiration provides a plant with the energy needed for
various plant physiological and morphological activities.
The general equation for respiration is:

C6H12O6 + 6O2 >>> 6CO2 + 6H2O + released
energy

Subtracting respiration from gross primary
production gives us net primary productivity (NPP),
which represents the rate of production of biomass that
is available for consumption (herbivory)
by heterotrophic organisms
(bacteria, fungi, and animals).

Globally, patterns of primary productivity
vary both spatially and temporally. The least productive
ecosystems are those limited by heat
energy and water like the deserts and the polar
tundra. The most productive ecosystems are systems with
high temperatures, plenty of water and lots of available
soil nitrogen. Table 9l-1 describes the approximate
average net primary productivity for a variety of ecosystem
types.

Table 9l-1: Average
annual Net Primary Productivity of the Earth's major
biomes.